In ionographic devices such as that described by U.S. Pat. No. 4,524,371 to Sheridon et al. or U.S. Pat. No. 4,463,363 to Gundlach et al., an ion producing device generates ions to be directed past a plurality of modulation electrodes for deposit on a charge retentive surface of an imaging member in imagewise configuration. In one class of ionographic devices, ions are produced at a coronode supported within an ion chamber, and a moving fluid stream entrains and carries ions produced at the coronode out of the chamber. At the chamber exit, a plurality of control electrodes or nibs are modulated with a control voltage to selectively control passage of ions through the chamber exit. Ions directed through the chamber exit are deposited on the charge retentive surface in imagewise configuration to form an electrostatic latent image developable by electrostatographic techniques for subsequent transfer to a final substrate. The arrangement produces a high resolution non-contact printing system. Other ionographic devices exist which operate similarly, but do not rely on a moving fluid stream to carry ions to a surface, such as, for example, U.S. Pat. No. 4,675,703 to Fotland or U.S. patent application Ser. No. 07/887,943 by Bergen et al.
One problem affecting the control of image quality in ionographic devices is known as "focusing". Focusing is an unavoidable phenomenon caused by a local distortion in an ionographic head design at the ion exit aperture in heads such as those described in U.S. Pat. No. 4,972,212 to Hauser et al., U.S. Pat. No. 4,973,994 to Schneider, U.S. Pat. No. 4,996,425 to Hauser et al., U.S. Pat. No. 5,081,476 to Genovese, U.S. Pat. No. 4,524,371 to Sheridon et al., U.S. Pat. No. 4,463,363 to Gundlach et al., U.S. Pat. No. 4,675,703 to Fotland and others. Generally speaking, and with reference to FIG. 1, the equipotential surfaces of the electrostatic projection field in the gap between the head and the charge retentive surface, that serve to electrostatically propel ions towards the surface, are generally planes oriented parallel to the charge retentive surface. However, at the ion exit aperture, the equipotential surfaces follow the profile of the head structure, and deviate from planes which changes the orientation of the local projection fields, as shown in FIG. 1. As ions are propelled through the projection field from the printhead aperture towards the imaging member, the field distortions at the aperture cause the ion paths to converge slightly, concentrating the ion flux into a narrower beam in the slow scan direction (the direction of relative movement between charge retentive surface and head). Thus, ions are "focused" into a smaller than desired area on the charge retentive surface. The resulting image artifacts are observed as reductions in the width of printed thin lines aligned at right angles to the fast scan direction, foreshortened characters, out-of-round dots that should be circular, and the like.
The problem exists with many types of ionographic devices, including but not limited to fluid jet assisted and non-fluid jet assisted ion devices, single apertured, and multi-apertured ion devices. An excessively narrow ion beam tends to make the charge depositing function of the printheads more sensitive to fluctuations in the process speed or motion quality of the electroreceptor.
The problem of blooming artifacts in ionographic printing has been addressed in U.S. Pat. No. 4,972,212 to Hauser, U.S. Pat. No. 4,973,994 to Schneider, and other patents. Blooming, an ionographic printing artifact resulting from previously deposited ions electrostatically repelling ions in the process of being deposited, is unidirectional with displacements in the direction opposite to surface velocity (the reverse process or reverse slow scan direction). Focusing tends to be symmetric, and in the case of an ionographic head with a slit aperture, reduces beam width parallel to the process direction.